Excimer lasers are well known. An important use of excimer lasers is to provide the light source for integrated circuit lithography. The type of excimer laser currently being supplied in substantial numbers for integrated circuit lithography is the KrF laser which produces ultraviolet light at a wavelength of 248 nm. A similar excimer laser, the ArF laser, provides ultraviolet light at 193 nm. These lasers typically operate in a pulse mode at pulse rates such as 1,000 Hz to 4,000 Hz. The laser beam is produced in a laser chamber containing a gain medium created by the discharge through a laser gas between two elongated electrodes of about 28 inches in length and separated by about 5/8 inch. The discharge is produced by imposing a high voltage such as about 20,000 volts across the electrodes. For the KrF laser, the laser gas is typically about 1% krypton, 0.1% fluorine and about 99% neon. For the ArF laser the gas is typically about 3 to 4% argon, 0.1% fluorine and 96 to 97% neon. In both cases in order to achieve high pulse rates of about 1,000 Hz to 4,000 Hz, the gas must be circulated between the electrodes at speeds of about 500 to 2,000 inches per second.
Fluorine is the most reactive element known and it becomes even more reactive when ionized during the electric discharge. Special care must be exercised to utilize in these laser chambers materials such as nickel coated aluminum which are reasonably compatible with fluorine. Further, laser chambers are pretreated with fluorine to create passification layers on the inside of the laser chamber walls. However, even with this special care, fluorine will react with the walls and other laser components which results in a relatively regular depletion of the fluorine. The rates of depletion are dependent on many factors, but for a given laser at a particular time in its useful life, the rates of depletion depend primarily on the pulse rate and load factor if the laser is operating. If the laser is not operating, the depletion rate is substantially reduced. The rate of depletion is further reduced if the gas is not being circulated. To make up for this depletion, new fluorine is typically injected at intervals of about 1 to 3 hours. Rather than inject pure fluorine it is a typical practice to inject into KrF lasers a mixture of 1% fluorine, 1% krypton and 98% neon. For example, in a typical 1000 Hz KrF excimer laser used for lithography, the quantity of its fluorine, krypton, neon mixture injected to compensate for the fluorine depletion varies from about 10 scc per hour when the laser is not operating and the laser gas is not being circulated to about 500 scc per hour when the laser is running continuously at 1000 Hz. The typical injection rate is about 120 scc per hour when the chamber fan is circulating the laser gas, but the laser is not firing. The unit "scc" refers to "standard cubic centimeters". Other commonly used units for describing quantities of fluorine in a particular volume are percent (%) fluorine, parts per million and kilo Pascals; the latter term sometimes refers to the partial pressure of the fluorine gas mixture. (This is because the amount of fluorine injected into a laser chamber is typically determined (directly or indirectly) by the measured chamber pressure increase while the 1% fluorine gas mixture is being injected.) A 320 scc per hour injection rate of the 1% fluorine mixture would correspond to a depletion in the fluorine concentration over 2 hours from about 0.10 percent to about 0.087 percent. The actual quantity of fluorine depleted in two hours as measured in grams of pure fluorine would be about 10 milligrams during the two hour period corresponding to the above 320 scc/hour injection rate (i.e., 640 scc of the 1% fluorine mixture injected at two-hour intervals) of the fluorine gas mixture.
For integrated circuit lithography a typical mode of operation requires laser pulses of constant pulse energy such as 10 mJ/pulse at about 1000 Hz to 4000 Hz which are applied to wafers in bursts such as about 300 pulses (a duration of about 300 milliseconds at 1,000 Hz) with a dead time of a fraction of a second to a few seconds between bursts. Modes of operation may be continuous 24 hours per day, seven days per week for several months, with scheduled down time for maintenance and repairs of, for example, 8 hours once per week or once every two weeks. Therefore, these lasers must be very reliable and substantially trouble-free.
In typical KrF and ArF excimer lasers used for lithography, high quality reproducible pulses with desired pulse energies of about 10 mJ/pulse may be obtained over a substantial range of fluorine concentration from about 0.08 percent (800 parts/million or about 24 kPa partial pressure of the fluorine gas mixture) to about 0.12 percent (1,200 parts/million or about 36 kPa).
Over the normal laser operating range the discharge voltage required to produce the desired pulse energy increases as the fluorine concentration decreases (assuming other laser parameters remain approximately constant). FIG. 1 shows the typical relationship between discharge voltage and fluorine concentration for constant pulse energy of 10 mJ and 14 mJ. The discharge voltage in the range of 15 kv to 20 kv is typically controlled by a feedback system which calculates a charging voltage (in the range of about 550 volts to 800 volts) needed to produce (in a pulse compression-amplification circuit) the discharge voltage which is needed to produce the desired laser pulse energy, the discharge voltage being roughly proportional to the charging voltage. This feedback circuit therefore sends a "command voltage" signal in the range of (550 volts to 800 volts) to a power supply to provide charging voltage pulses.
Prior art techniques typically utilize the relationship between discharge voltage and fluorine concentration to maintain constant pulse energy despite the continuous depletion of fluorine. The discharge voltage of prior art excimer lasers can be changed very quickly and accurately and can be controlled with electronic feedback to maintain constant pulse energy. Accurate and precise control of fluorine concentration in the past has proven difficult. Therefore, in typical prior art KrF and ArF laser systems, the fluorine concentration is allowed to decrease for periods of about 1 to 4 or 5 hours while the charging voltage and thus the discharge voltage is regulated by a feedback control system to maintain approximately constant pulse energy output. Periodically at intervals of about 1 to a few hours, fluorine is injected during short injection periods of a few seconds. Thus, in normal operations fluorine concentration gradually decreases from (for example) about 0.10 percent to about 0.09 percent over a period of about 1 to a few hours while the discharge voltage is increased over the same period from for example about 600 volts to about 640 volts. The injection of fluorine at the end of the 1 to a few hour period (when the voltage has drifted up to about 640 volts) brings the fluorine concentration back to about 0.10 percent and the feedback control (maintaining constant pulse energy) automatically reduces the voltage back to 600 volts. This basic process is typically repeated for several days. Since contamination gradually builds up in the laser gas over a period of several days, it is usually desirable to replace substantially all of the gas in the laser with new laser gas at intervals of about 5-10 days. FIG. 2 describes the prior art fluorine injection technique discussed above. The voltage values represent average values of control voltage commands and indirectly represent average values of the actualcharging voltage.
The above-described prior art technique is effectively used today to provide long term reliable operation of these excimer lasers in a manufacturing environment. However, several laser parameters, such as bandwidth, beam profile and wavelength, are adversely affected by the substantial swings in the discharge voltage and fluorine concentration.
A substantial number of techniques have been proposed and patented for measuring and controlling the fluorine concentration in excimer lasers to within more narrow limits than those provided under the above described prior art technique. These techniques have generally not been commercially pursued.
In some situations an integrated circuit lithography may use a particular resist requiring much less energy for curing than normal. For example, photo resists on the average may require 50 mJ/cm.sup.2 for curing, but a special resist may require only 20 mJ/cm.sup.2. A simple technique would be to merely reduce the number of pulses each spot on the wafer sees. For example, if 40 pulses at 10 mJ per pulse were required to provide 50 mJ/cm.sup.2 on the wafer, then 16 pules would provide the 20 mJ/cm.sup.2 for the very sensitive resist. However, as the number of pulses decreases total dose stability increases which is undesirable. Other solutions are to keep the 40 pulses but to either reduce the pulse energy output or to attenuate the beam. Reducing the pulse energy output of a typical prior art laser below its optimal energy usually results in reduced operating efficiency and, of course, attenuating the beam by definition would reduce the overall efficiency of the system.
What is needed is an economical method of operating a lithography laser with energy efficiency in a lithography environment in which resists with a substantial range of